Fluidic devices can have integrated fluid channels for directing and controlling the transport of fluids. Microfluidics, a miniaturized form of fluidics, has emerged as a new approach for improving the performance and functionality of such systems for chemical and biochemical synthesis, as well as chemical, biochemical, and medical analysis. Miniaturization and new effects in micro-scale promise completely new system solutions in these fields. Dimension reduction results in faster processes with reduced reagent and sample consumption rates. The small size scale also encourages parallel processing, in which more compounds can be produced and/or analyzed simultaneously. Massively parallel processing can speed DNA, RNA, protein, immunologic, and other tests to reduce time intervals for drug discovery and medical diagnosis. Currently, microfluidic based microanalysis systems for such applications typically have fluid channel dimensions on the order of tenths of millimeters to several millimeters, although future trends are to further reduce channel dimensions. Various microfluidic components have also been demonstrated on the same size scale, for example: micro-valves, micro-pumps, micro-flow sensors, micro-filters, micro-mixers, micro-reactors, micro-separators, and micro-dispensers, to name just a few. The book, FUNDAMENTALS AND APPLICATIONS OF MICROFLUIDICS by Nam-Trung Nguyen and Steven T. Werely, published by Artech House of Boston, U.S.A., in 2002 provides an overview of some microfluidic technologies and applications. FIGS. 1A and 1B illustrated exploded and assembled perspective views of a fluidic or microfluidic device composed of a body component 101 to which a cover component 103 is affixed. Body component 101 contains channels 102 and other fluidic or microfluidic components formed therein.
In fluidic systems, and microfluidic systems in particular, it is often desirable to mix fluid streams prior to subsequent processing. Merging separate fluid streams in a uniform and reproducible manner in fluidic systems, and in particular in microfluidic systems can be difficult. Often, one fluid stream may reach the merging point earlier than another due to unmatched flow impedances or driving forces. Sometimes an air bubble can be trappedin the merged stream. This can be problematical if downstream operations require a homogeneous liquid media, free of entrapped bubbles. For example, if the downstream process is a DNA microarray hybridization, the presence of air bubbles could obstruct the binding of target molecules in a buffer with immobilized probes on the microarray.
U.S. Pat. No. 6,601,613, issued to McNeely et al. on Aug. 5, 2003 (hereinafter “McNeely”), teaches the use of passive valves formed in microfluidic channels by reducing channel width and/or height to help overcome the problem discussed above. The passive valves act as barriers to impede the flow of solution until enough force is built up to overcome the force the pressure barrier. Flow through multiple channels can be regulated to allow a series of sister wells or chambers to fill prior to the fluid flowing beyond. A hydrophobic vent structure is introduced to allow entrapped bubbles to escape subsequent to merging. For such mechanism to work well, the initially opened vent needs to be closed by the time it is passed by the fluid tails; otherwise, the vent could leak fluid in many operational situations. Therefore, in general the hydrophobic vent needs to be used in conjunction with a feedback control system comprising an active valve, as well as a sensor to open the active valve for bubble venting, but to keep the valve closed at other times. Another disadvantage is that additional steps are required during device fabrication in order to render the gas vent channel hydrophobic in substrates often made of hydrophilic materials such as polymethylmethacrylate (PMMA), polycarbonate (PC) or glass. An additional difficulty of this approach is that uniformly creating passive valves by reducing microfluidic channel width necessitates tighter dimensional tolerances for the microfluidic device, reducing yield and adding to fabrication cost.